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Energy in a Cell
CHAPTER 9
Cell Energy
Energy is essential to life
Plants trap light energy in sunlight and store it in the
bonds of certain molecules to use later
Other organisms get energy from eating those green
plants
What processes can you name that require energy?
Adenosine Triphosphate (ATP)
 An energy molecule in the cell that allows for quick
and easy access to energy when needed by the cell’s
organelles.
 A type of chemical energy
 Releases energy when the chemical bonds are broken
 A-P-P-P
Forming ATP
 Phosphate groups are negatively charged
 Negative doesn’t like being next to negative
 A small amount of energy is required to attach one
phosphate group to adenosine (AMP)
 When a second phosphate group is added, this
requires a lot more energy (ADP)
 When a third phosphate group is added, this requires
an even greater amount of energy (ATP)
 The process of forming ATP requires much energy
Breaking down ATP
 Energy of ATP becomes available to a cell when the
molecule is broken down
 When a cell requires energy, ATP goes to the cell,
attaches to the binding site, and a phosphate group is
broken off – this gives off energy for the cell and the
ATP molecule becomes ADP (fig. 9.2 pg. 223)
 In order for ADP to become ATP again it goes to the
mitochondria and gets recharged (another
phosphate group gets attached)
Uses of cell energy
 Energy is VERY important on the cellular level
 Making new molecules
 Building membranes and cell organelles
 Cells use energy to maintain homeostasis
 Kidneys use energy to move molecules and ions in order to
eliminate waste substances while keeping needed substances
in the bloodstream.
Photosynthesis
 In this section…
 What is photosynthesis?
 Where photosynthesis happens
 Color: How it works
 The two phases of photosynthesis
Photosynthesis
 A process of taking light energy and converting it
into chemical energy
 This energy is stored as carbohydrates in plants
 Happens in two phases:
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
Light-dependent reactions- converts light energy into chemical
energy-molecules of ATP produced fuel light-independent
reactions
Light-independent reactions- produce glucose
Where does photosynthesis occur?
 In chloroplasts there are thylakoid disks/grana
 Light-dependent reactions happen in the thylakoid
membranes
Pigments
 To trap the energy in the sunlight, the thylakoid
membranes contain pigments.
 Pigments are molecules that absorb specific
wavelengths of sunlight
 Chlorophyll is the most common type of pigment in
chloroplasts
Why we see color
 We see colors that are reflected and not absorbed.
Green pigments absorb all light except green
(chlorophyll)
 In the fall, trees stop producing chlorophyll, which
results in the different colors seen.
The Big Picture
Energy
Light-Dependent Reactions
Light-Independent Reactions
(Calvin Cycle)
Stored Energy
(stored as glucose)
Light-Dependent Reactions
 First phase of photosynthesis requires sunlight.
 A light-dependent reaction involves sunlight striking
molecules of chlorophyll and exciting an electron.
 Excited electrons are passed from chlorophyll to an
electron transport chain
 Electron transport chain- a series of proteins
embedded in the thylakoid membrane
Light-dependent reactions
 Once in the electron transport chain, each protein in
the chain passes the energized electrons along to the
next protein some energy is lost during each pass lost energy can be used to form ATP from ADP OR
to pump hydrogen ions into the center of the
thylakoid disc.
 After electrons have traveled down the electron
transport chain, they are re-energized in a second
photosystem and passed down a second electron
transport chain- electrons are still very energized
Light-Dependent reactions
 Electrons are then transferred to the stroma of the
chloroplast
 Transferred by an electron carrier molecule called
NADP+ (nicotinamide adenine dinucleotide
phosphate)
 NADP+ can combine with two excited electrons and
a hydrogen ion (H+) to become NADPH.
 NADPH stores the energy until it can transfer it to
the stroma- this is where NADPH will play an
important role in the light-independent reaction.
Restoring Electrons
 The chlorophyll needs to replace the electrons that were
lost at the beginning of photosynthesis in order to absorb
additional light to keep the process going.
 To replace lost electrons, the molecules of water are split
in the first photostem- reaction called photolysis
 For every water molecule that is split, 1 Oxygen, 2
electrons, and 2 Hydrogen ions are formed



Oxygen produced by photolysis is released into the air-supplies
oxygen for air we breathe
Electrons are returned to the chlorophyll
H+ ions are pumped into the thylakoid-> they accumulate in high
concentrations which causes a concentration gradient- H+ ions
diffuse out of thylakoid and provide energy for production of ATP
(called chemiosmosis)
Light-Independent reactions
 2nd phase of photosynthesis
 Does NOT require light
 Takes place in the stroma of the chloroplast
 Aka Calvin cycle- called a cycle bc one of the
products is needed to start the cycle over
 Follow the cycle on pg. 229
The Calvin Cycle
 1) CARBON FIXATION-The carbon atom from CO2 bonds with a five-




carbon sugar called ribulose biphosphate (RuBP) to form an unstable six
carbon sugar.
2) FORMATION OF 3-CARBON MOLECULES-The six-carbon sugar
immediately splits to form two three-carbon molecules.
3) USE OF ATP AND NADPH-A series of reactions involving ATP and
NADPH from the light-dependent reactions converts the three-carbon
molecules into phosphoglyceraldehyde (PGAL), three-carbon sugars with
higher energy bonds.
4) SUGAR PRODUCTION- One out of every six molecules of PGAL is
transferred to the cytoplasm and used in the synthesis of sugars and other
carbohydrates. After three rounds of the cycle, six molecules of PGAL are
produced.
5) RuBP IS REPLENISHED- Five molecules of PGAL, each with three
carbon atoms, produce three molecules of the five-carbon RuBP. This
replenishes the RuBP that was used up, and the cycle can continue.
Getting Energy to Make ATP
 Cellular Respiration- The process by which
mitochondria break down food molecules to produce
ATP.
 There are 3 stages of cellular respiration



1) Glycolysis- anaerobic (no oxygen required)
2) Citric acid cycle- aerobic (oxygen required)
3) Electron transport chain- aerobic (oxygen required)
3 parts of cellular respiration
Glycolysis
 a series of reactions in the cytoplasm of a cell in
which glucose (a 6 carbon molecule) is broken
down into two molecules of pyruvic acid (3 carbon
molecules).
 ATP - it takes 2 molecules of ATP to start the
process of glycolysis, and only 4 ATPs are made,
therefore this process is not very energy efficient.
Glycolysis
 *only 2 molecules of ATP are produced from the
breakdown of one glucose molecule.
 NAD+ (nicotinamide dinucleotide) - just as
photosynthesis has the energy carrier NADP+;
glycolysis has an energy carrier called NAD+.
 *NAD+ forms NADH when carrying an electron.
 At the end of glycolysis the pyruvic acid molecules
produced move to the mitochondria,the
powerhouses or ATP producers of the cell.
Glycolysis
Glycolysis- on a molecular level
Post-Glycolysis
 Post-glycolysis reactions - before the pyruvic acid
molecules can enter the citric acid cycle (the next
stage of cellular respiration) some modifications
need to be done.


pyruvic acid loses a molecule of CO2 and combines with
Coenzyme A to form a molecule of Acetyl-CoA.
the rxn w/ Coenzyme A makes a molecule of NADH+ H+
The Citric Acid Cycle
The Citric Acid Cycle: “The breakdown of Glucose”-a
series of chemical reactions similar to the Calvin Cycle,
but opposite in purpose.
Calvin Cycle - forms glucose in photosynthesis
Citric Acid Cycle - breaks down glucose in cellular
respiration
Materials needed :
to break down glucose, two electron carriers are
needed:
a) NAD+
b) FAD (flavin adenine dinucleotide)
The Citric Acid cycle
The Citric Acid Cycle (CAC) produces a number of
molecules:
a) 1 ATP is produced
b) 3 NADH + H+ are produced
c) 1 FADH2 molecule is produced
Steps of the Citric Acid Cycle
1) formation of citric acid - a 2 carbon acetyl CoA combines with a 4
carbon compound called oxaloacetic acid, forming a 6 carbon molecule
called citric acid.
2) formation of CO2 - one molecule of CO2 is formed from the citric
acid cycle which reduces the citric acid molecule to a 5 carbon molecule
called ketoglutaric acid.*from this rxn, one molecule of NADH +H+ is
made from one NAD+
3) formation of second CO2 - another molecule of CO2 is formed and
released from the ketoglutaric acid; this results in a 4 carbon
compound called succinic acid. *from this rxn, one molecule of ATP and
one molecule of NADH + H+ are formed.
4) recycling of oxaloacetic acid - succinic acid undergoes a series of
rxns which form FADH and NADH + H+ and oxaloacetic acid; this is
then available for the next cycle to occur.
Succinic -> fumaric -> malic -> oxaloacetic
Citric Acid Cycle
Citric Acid Cycle
Electron Transport Chain
 Function - move energized molecules; NADH &
FADH2 pass energized molecules from protein to
protein releasing small amounts of energy with
each pass.
 Location - the inner membrane of the
mitochondria
Mitochondria
Electron Transport Chain
The Process:
a) NADH & FADH2 pass energized molecules from protein to
protein; small amounts of energy are released with each
pass.
b) some energy is used to form ATP, while some is used to
pump H+ ions into the center of the mitochondria.
c) as H+ ions are pumped into the center of the mitochondria,
the center becomes more (+),while the outside becomes
more (-). Since the outside is more (-) it will attract more
(+)’s or more H+ ions,creating an electrochemical gradient.
d) The electrochemical gradient drives the inner membrane
of the mitochondria to form ATP.
e) The final electron acceptor in the ETC is Oxygen. The
oxygen reacts with H+ ions to form water molecules.
ETC in mitochondria
ETC Importance
The importance of Oxygen (O2)
If oxygen is not available for the ETC, then the chain
cannot pass along energized electrons; if electrons cannot
be passed, then there is no room to accept more electrons
and a blockage results. Therefore, cellular respiration
cannot occur.
Overall production
 The ETC results in the production of 32 ATP molecules
 This is the most efficient means for production of ATP
 Think: Aerobic (jogging) vs, Anaerobic (sprinting) - which
can be done longer?
Fermentation
 sometimes your cells may be deprived of oxygen for a
short time
 fermentation can occur during extremely strenuous
activities
 Fermentation - anaerobic process that occurs when
your cells are w/o O2 for a short time. It occurs after
glycolysis and provides a way to continue producing
ATP until oxygen is available again.
 2 main types of fermentation:
a) lactic acid fermentation
b) alcoholic fermentation
Lactic Acid Fermentation
*occurs during anaerobic conditions when oxygen is not
available as the final electron acceptor in the ETC,
therefore a “back-up” occurs.
What happens:
a) as NADH and FADH2 try to pass their
energized electrons onto the next protein in the
ETC, they are rejected.
b) if NADH and FADH2 cannot pass on their
energized electrons, then NADH and FADH2
cannot be converted back to NAD+ & FAD, which
are needed to keep the CAC and glycolysis
going.
Alcoholic Fermentation
*often used by yeast cells to produce CO2 and ethyl
alcohol.
*anaerobic process - used to make bread dough “rise”
and brew alcohols.
Comparing Photosynthesis and Cellular
Respiration
 Both use an ETC to form ATP
 Do opposite jobs
 Photosynthesis - produces high energy carbohydrates
and O2 from the sun’s energy
 Cellular respiration - uses O2 to break down
carbohydrates with much lower energy level
Comparisons:
Photosynthesis
Food is made or
accumulated
Cellular Respiration
Food is Broken down
Energy from sun is stored as Energy from glucose is
glucose
released to be used by body
Carbon dioxide (CO2) is
taken in
CO2 is given off as a waste
product
Oxygen (O2) is given off as
waste
Oxygen is needed and is
taken in
Produces glucose from
PGAL
Produces CO2 and H2O as
waste
Can happen only when
some light is available
Can occur all day and all
night
Requires Chlorophyll, can
only happen in plants
Occurs in all living cells plants and animals